Lindsey Graham, a Republican senator for South Carolina, and Sheldon Whitehouse, a Democrat senator for Rhode Island, jointly signed a letter on Wednesday addressed to both organisations, requesting clarification.

“The president has asked congress to look into whether or not his campaign was wiretapped by the Obama administration,” said Mr Graham, shortly after his letter was made public. “I will take up that challenge.”

“I expect them to come forward as to whether or not a warrant was obtained.”

Asked if he would subpoena this information, if not willingly provided, he replied: “Yes.”

The pair have asked James Comey, FBI director, and Dana Boente, the acting deputy attorney general, to respond. Mr Boente is in charge of any potential investigation after Jeff Sessions, the attorney general, recused himself following revelations of a meeting with the Russian ambassador which he failed to disclose.

“We would be equally alarmed to learn that a court found enough evidence of criminal activity or contact with a foreign power to legally authorize a wiretap of President Trump, the Trump Campaign, or Trump Tower.

“We request that the Department of Justice provide us copies of any warrant applications and court orders — redacted as necessary to protect intelligence sources and methods that may be compromised by disclosure, and to protect any ongoing investigations.”

James Comey, director of the FBICredit:
Rex

Mr Comey on Wednesday spoke at a cybersecurity conference in Boston – refusing to take questions, and not discussing the specifics of wiretapping.

But Mr Comey was reportedly “incredulous” about Mr Trump’s accusations, and said Mr Obama did not authorise any wiretaps. The FBI director has reportedly asked the justice department to publicly refute Mr Trump's accusations – something they are yet to do.